Optical transmission line elements utilizing fluorinated polymers

ABSTRACT

Optical transmission lines constructed of fully fluorinated polymers in the amorphous state, such as fluorinated ethylene propylene, have low inherent insertion loss characteristics.

United State:

Pinnow et a].

[ OPTICAL TRANSMISSION LINE ELEMENTS UTILIZING F LUORINATED POLYMERS[75] Inventors: Douglas Arthur Pinnow, Berkeley Heights; LeGrand GerardVan Uitert, Morris, both of NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill. Berkeley Heights, NJ.

[22] Filed: May [7. 1971 [2]] Appl. No.: 143,876

[52] US. Cl 350/96 WG, 350/96 R [51] Int. Cl. G02b 5/I4 [58] Field ofSearch 350/96 WG, 175 NO [56] References Cited UNITED STATES PATENTS3.556,635 l/l97l Schrenk et a]. 350/96 WG UX 3,64l 332 2/l972 Reick etal 350/96 R X l b i l i 2.794.959 6/1957 Fox 350/96 wo ux 3,542,46111/1970 Girard 35l/l6.0 3,386,787 6/1968 Kaplan 350/96 wo FOREIGNPATENTS OR APPLICATIONS 1,037.498 7/1966 Great Britain 350/96 wo OTHERPUBLICATIONS Marcuse et al., Mode Conversion Caused by Diameter Changesof a Round Dielectric Waveguide The Bell System Technical Journal, Vol.48, No. 10. Dec. 1969, pp. 3217-3232.

Primary ExaminerJohn K. Corbin Anorne vW. L. Keefauver and Edwin B. Cave[57] ABSTRACT Optical transmission lines constructed of fullyfluorinated polymers in the amorphous state, such as fluorinatedethylene propylene, have low inherent insertion loss characteristics.

9 Claims, 1 Drawing Figure OPTICAL TRANSMISSION LINE ELEMENTS UTILIZINGFLUORINATED POLYMERS BACKGROUND OF THE INVENTION 1. Field of theInvention The invention is concerned with glass transmission lines foruse in the visible and near-visible spectrum.

2. Description of the Prior Art The invention of the laser almostimmediately prompted interest in the development of broadbandcommunications systems. Progress has been significant. New and moreefficient lasers have evolved as have useful circuit elements performinga multitude of functions, e.g., modulation, frequency shifting,isolation, etc. It is well known to workers in the field that asignificant obstacle to a light communication system is the developmentof a suitable low-loss transmission medium. Various approaches have beenpursued; some focusing, some not focusing, some utilizing vacuum, someusing gaseous media, some crystalline or glassy. Of these, many workersbelieve the glass transmission line to be most promising, particularlyfor intrametropolis and other short-haul use.

Probably the most significant work in glass transmission lines has beenconcerned with various types of silicate glasses. Such materials arefamiliar; preparatory techniques are known, and they are possessed ofcertain obvious practical advantages, e.g., chemical and physicalstability. See, for example, Vol. II, Glass Technology, pp.30-35, AprilI970. According to F. P.

-Kapron et al., Applied Physics Letters, volume l7, pp.

423-425, Nov. 15, 1970, transmission lines of carefully prepared fusedsilica have insertion losses of approximately 20 dB per kilometer at awavelength of 6,328 angstrom units.

It is reasonable to assume that the best of the optical transmissionmaterials now considered to be of greatest interest will soon beavailable in such a high degree of perfection that insertion loss willbe due, in large part, to inherent characteristics. It may be estimatedthat pure silica in its most perfect form will have an insertion loss ofapproximately 5 dB/km at a wavelength of 0.5 microns which is near thecenter of the visible spectrum.

SUMMARY OF THE INVENTION In accordance with the invention, opticaltransmission lines are constructed of fiuorinated organic polymericmaterial in a glassy state. While processing complications may beencountered in minimizing crystallinity for certain of the includedcompositions, the additional effort is justified on the basis of anextremely small inherent loss.

Use of fiuorinated polymers, exemplified by tetrafiuorinatedethylene-propylene copolymer is prompted by a theoretical developmentreported herein. Accordingly it is determined that inherent scatteringloss is proportional to the eighth power of the refractive index.Indices of materials of the invention are generally below about 1.35relative to vacuum. These are among the lowest indices of knownnongaseous media and compare with values of about 1.5 for silicaglasses.

Other properties of the fluorocarbon materials of the invention aregenerally desirable. Such materials are substantially unaffected byusual atmospheric conditions and otherwise show requisite chemical andphysical stability for transmission line use. Once crystallinity hasbeen minimized, materials are readily formed into fibers or othershapes, either self-supporting or supported.

Other favorable characteristics concerned with melting point, absorptionrelated to energy gap, and absorption due to infrared active mechanicalmodes are discussed under the Detailed Description.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a cross-sectional viewofa portion of a glass transmission line in accordance with theinvention.

DETAILED DESCRIPTION l. The Drawing The FIGURE depicts a portion of anoptical transmission line I constructed of a fiuorinated glassy polymerin accordance with the invention. Electromagnetic radiation depicted asarrows 3 and 4 is introduced from source and by means not shown and isextracted by means not shown. Introduction and extraction means may bothinclude closely matched or otherwise related media of appropriaterefractive index and/or may include one or more optically polishedsurfaces. For general purposes, radiation to be transmitted in theinventive structure is either incoherent or coherent so that anappropriate source may constitute any of the variety of light-emittingdiodes or laser oscillators which emit radiation which is, itself, of awavelength within the transparent region of medium 2 or which may beconverted by parametric or other means to fall within this region.

Structure 1 may be regarded as self-supporting, for example, in the formof a fiber or it may be supported on a substrate, in which event, it mayconstitute a segment of a printed optical circuit.

2. Theory It is well known that an optical beam exponentially decreasesin intensity when traveling through an atten' uating medium. Thus, abeam ofinitial intensity I is reduced to intensity I after traveling adistance X. Thus where a is the total attenuation coefficient which iscomposed of two parts-associated with scattering and absorption m: uralnbo The first part is the Rayleigh scattering coefficient. A recentanalysis of the Rayleigh scattering loss in optical glass (Pinnow,unpublished) indicates that a is proportional to the following materialparameters:

lrul where n index of refraction p photoelastic component p density Vsound velocity T ambient temperature and T, glass melting temperature.

This analysis is based on an extension of classical light scatteringtheory (see, for example, I. L. Fabelinskii, Molecular Scattering ofLight, Plenum Press, New York; [968) modified by the recent theory of D.A. Pinnow, S. J. Candau, .l. T. LaMacchia, and T. A. Litovitz (JournalofAcousticaI Society of America, Vol. 43, I3 l-l42, Jan. 1968) which isspecifically applicable to the glassy state.

The most important parameters in this formula are n" and T The magnitudeofp for most liquids and glasses can be approximated by theLorentz-Lorentz value which is almost constant and equal to 0.35 for nin the range of 1.5 to 2.5 (for indices of refraction less than 1.5, pis somewhat smaller), see D. A. Pinnow, IEEE Journal of QuantumElectronics, Vol. QE-b, 223-238, April 1970). The quantity V whichappears in the denominator of the Rayleigh scattering formula is equalto the elastic modules of a material which may vary considerably fromone material to the next. However, it is expected that the variations inn and T will dominate the Rayleigh scattering coefficient. In order toreduce this scattering, materials should be selected with low n(T+T,,)values.

From the inventive standpoint, primary significance is attached to theinfluence of refractive index on loss due to inherent scatteringmechanisms. It has certainly been recognized previously that scatteringis reduced for lowered refractive index, but the fact that thedependence is on the index raised to the eighth power is surprising. Thesignificance of this factor in the insertion loss gives impetus to thesearch for otherwise suitable low index materials. This impetus issufficient to focus attention on the generally overlooked materials ofthe invention which in their usual form, may not be obviously suitablefor light transmission use.

The materials of the invention are advantageous also from the standpointof low 'I',,. Typically, melting points for the materials of theinvention are of the order of 350 C which compares quite favorably withthe usual silicate glasses having melting points of the order of l000 Cor higher (melting point is here defined as the temperature to which themedium must be raised to reduce its viscosity to the value of aboutpoises). Other advantages are discussed in a succeeding section.

3. Composition As discussed in some detail in the next section, primaryadvantages ascribed to the materials utilized in the structures of theinvention derive from carbonfluorine bonding. An overriding requirementfor all included compositions is, therefore, expressed in terms of thenumber of such bonds. Advantages ascribed to the included materialsaccrue as the ratio of carbon-tofluorin'e bonds relative to othercarbon-to-terminal atom bonds increases. Such advantages, for example,including lowered refractive index, are maximized for a totallyfluorinated polymer. The minimal requirement based on suchconsiderations is a ratio of at least 50 percent, it being consideredthat substantially smaller ratios result in indices and othertransmission characteristics representative only of relativelyinsubstantial improvement as compared with nonfluorinated polymers.Other compositional considerations are dictated where the polymer is nottotally fluorinated so that some of the carbon-to-terminal atom bondsinvolve other elements. To minimize absorptions due to hydrogen(typically at wavelengths of about 3.4, L7, and LI micrometers), it is ageneral requirement that the ratio of CH bonds to the totallity of bondsbetween carbon atoms and terminal atoms be kept to a minimum. From thestandpoint of the invention, it is generally desired that such ratio beno greater than about 1:3 with a preference existing for l:l0 andoptimally a ratio of less than l:l0

Where the polymeric material is not totally fluorinated, it is desiredthat terminal atoms be deuterium since the characteristic infraredabsorptions are shifted to about 4.8, 2.4, and L6 micrometers. Inaddition to avoiding the shorter wavelength absorption associated withhydrogen, the deuterium atoms may be so placed as to give rise to a netdipole moment associated with the CF bonds. Molecular alignment whichmay be achieved under such circumstances by means of electrical polingwith or without mechanical working may result in reduced scattering.

Other terminal groups are permitted providing carbon bonding to suchgroups is kept below the maximum prescribed above; of course, providingthat such terminal groups are not of such nature as to havecharacteristic absorptions within a concerned wavelength range.

Generally, the most prevalent class of acceptable polymers are thefluorocarbons. As discussed above, the preferred subclass containsprimarily or solely fluorine terminal atoms. An exemplary material isthe copolymer of fully fluorinated ethylene propylene. In this preferredsubclass, the ratio of ethylene to propylene should be such as to bepolymerizable from intital ingredients which include at least [0 percentof each monomer. The advantage of the copolymer as compared to ahomopolymer, such as polytetrofluoroethylene, is the comparative easewith which the crystallinity may be minimized.

Another class of polymeric materials meeting the inventive requirementsis the perfluoroalkylpolyethers. These materials may be represented ashaving the formula:

The value of m in the preceding formula is of general interest. For thatparticular compound, values of m above about 10 are sufficient to resultin a reasonably rigid material at room temperature. Values below 10 downto about 3 are not to be discounted. Liquid state materials of theinvention are suitable guide materials and retain the noted advantages(and here again the distinction between rigid and liquid is found tooccur at a viscosity of approximately 10 poises). Liquid materials, inaccordance with the invention, have the advantage of further reducedRayleigh scattering due to the influence of T in the relationship setforth in Equation (I). Use of liquids also avoids difficulties due tocrystallinity. Of course, liquids require containing walls andprecautions against discontinuities which may develop during use.

In general, it is preferred that materials of the invention be rigid(i.e., having a viscosity above I0 poises) at the operating temperaturewhich is usually defined as room temperature.

4. Material Preparation Certain of the inventive materials tend tocrystallize. Crystallinity may be minimized in these materials by rapidcooling. ln general, quench rates of at least about l C per secondgenerally from the interval between a molten and rigid form aresufficient to meet this desired criterion. It is fortunate that thegenerally small diameters of the optical fibers permit such rapid cool-For liquid polymers the major requirement is simply absence ofimpurities or other inhomogeneities which may produce scattering orabsorption. Purities of 1 ppm are generally achievable and aredesirable.

5. Examples Examples may be divided into liquid or rigid polymers. Theliquids of interest include perfluoroalkylpolyethers. e.g.,

with m 1.20.

For example where m=9 the a is less than one-half that of fused SiOExceeding the limit increases a For example, when m was as high as 43 awas 10 times worse than fused silica. Other liquids of interest areBoiling Point Solids include materials with an average of more than 1000repetitive monomer formula units per chain (m 1000). Such materials canbe processed to maintain a preferred orientation as discussed earlier.The solids include polymers of tetrafluoro ethylene chlorotrifluoroethylene hexafluoro propylene octafluoro hutylene perfluoro propyleneoxide and mixtures and copolymers thereof.

Many variations are possible which may improve cross linkage betweenchains and thereby reduce crystallization and improve opticalproperties.

The requirement for molecule length may be set forth in general termsregardless of composition. Accordingly, it is specified that moleculesof particular interest contain either up to about 50 carbon atoms or,

alternatively, above about 2000 carbon atoms in amain polymer chain. Thefirst limit defines materials which are normally liquid at roomtemperature. The low limit is not specified and is to be determinedprimarily on the basis of permitted volatility for a given structure.Above about 50 carbon atoms the molecule length begins to approach theorder of magnitude of a wavelength of light. The range from about 50carbons to about 2000 carbons is to be avoided since dimensions,regardless of composition. are such as to result in an increase inscattering efficiency. The maximum for the normally rigid polymers(containing more than about 2000 carbons in the main chain) is to bedetermined on the basis of such practical considerations as processingdifficulty and availability. Materials having substantially largernumbers of carbons are useful providing the general requirements of theinvention are met, e.g., crystallinity below about 1 percent, etc.

6. General Properties The strong carbon-to-fluorine bond is responsiblefor the primary characteristics of the material to which the inventionis limited. The effect on refractive index is considered to arise fromthe constraint which is imposed on bonding electrons which, in turn,minimizes their contribution to polarizing effects. In fact, fluorinatedmaterials of the nature here described have indices of refraction whichrange generally from about 1.2 to about 1.35.

While the rigid fluorocarbons are noted for their high melting points,as compared with other organic polymeric materials, melting points aregenerally less than about 350 C. Materials utilized in accordance withthe invention are, therefore, temperature stable in any likelyenvironment to be encountered in operation but the melting point isstill sufficiently low to reduce this contribution to Rayleighscattering as compared with the usual inorganic glasses.

Another consequence of the strong C-F bond is the relatively largeenergy gap of the order of 6eV or greater. In consequence, completelyfluorinated fluorocarbons are quite transparent in the ultravioletportion of the spectrum (an energy gap of 6eV corresponds to awavelength of about 2000 angstrom units). While this may not be ofimmediate commercial interest since ultraviolet systems are notpresently at an advanced development stage, there are consequences atlonger wavelength. A contribution to optical absorption decreases inproportion to the difference in the transmitted wavelength and thewavelength corresponding with the energy gap. This contribution isconsidered to be related to absorptive transitions to localized stateswithin the forbidden energy gap which are thought to be fundamentallyassociated with amorphous structures. Regardless of the source of thiscontribution, it has been experimentally verified that absorption for agiven wavelength of transmitted energy decreases for increasing energygap.

An additional advantage of fluorinated polymeric glasses is increasedtransparency at longer infrared wavelength than for many othermaterials. This is, in turn, due to the fact that the characteristicbandstretching frequency of the carbon-to-fluroine bond is sustantiallyfurther out in the infrared spectrum than is that of other bonds (about80,000 angstrom units for C-F as compared with 34,000 angstrom units forC-H). Substitution of deuterium for hydrogen in partially fluorinatedpolymers also extends the infrared absorption edge out to about 48,000Angstrom units while infrared absorption in pure silicate glasses, bycomparison, sets in only at about 100,000 angstrom units, waterimpurities, which are difficult to remove, generally cause absorption atabout 27,000 angstrom units.

What is claimed is:

1. Optical transmission line of electromagnetic radiation for use withina spectrum including infrared and ultraviolet radiation comprising asubstantially amorphous member for transmitting such radiation and firstand second means for introducing and extracting radiation into and fromsaid member, respectively, characterized in that said member consistsessentially of a composition having a crystallinity of less than 1percent as measured by optical or X-ray techniques and in that the saidcomposition contains at least 65 percent carbon-to-fluorine bonds basedon the total number of bonds between carbon atoms and terminal atoms.

2. Transmission line of claim 1 in which said composition is polymeric,and has a viscosity of at least 10 poises at room temperature.

3. Transmission line of claim 2 in which said composition contains anaverage of at least 2000 carbon atoms in the main chain of the polymermolecule and is rigid at room temperature.

4. Transmission line of claim 1 in which essentially all the bondsbetween carbon atoms and terminal atoms are carbon-to-fluorine bonds.

5. Transmission line of claim 4 in which said composition consistsessentially of a fluorocarbon.

6. Transmission line of claim 5 in which said fluorocarbon isfluorinated ethylene propylene copolymer containing at least 10 percentof moieties which may be derived from each of the monomers, ethylene andpropylene.

7. Transmission line of claim 6 in which the average chain of saidcopolymer contains at least l0 monomeric units.

8. Transmission line of claim 4 in which said composition consistsessentially of perfluoroalkylpolyethers, in which the average polymerchain contains at least 10 monomeric units.

9. Transmission line of claim 1 in which said composition is normallyliquid and has an average of up to 50 carbon atoms per molecule.

YFIDRM Po-wso 16-69) UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,779,627 Daze December 18. 197% Invent r(s)Douglas A. Pinnow and LeGrand G. Van Uitert 'It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

*Golumn 2, line 56 change "(Pinnow, unpublished)" to read:

I v V --v(s ee an article entitled Fundamental Optical AttenuationLimitsinche Liquid and Glassy Statelwith Ajoplication to Fiber OpticalWaveguide Materials" by D. A. Pinnow etl' al appearing in. AppliedPhysics Letters, I M y 15, 1973 Column 3 Equation (l-l), change '(n =2)"to read" n +2)-. 76 01111115 6, line change "fluroine' to read"fluorine"; v

* Column 6', line 61%,. change "sustantially' to read I-substantially--.

Signed and sealed this 24th day of September 1974.

Attest: I McCOY M. GIBSON JR. c. MARSHALL DAN N Attesting OfficerCommissioner of v Patents

2. Transmission line of claim 1 in which said composition is polymeric,and has a viscosity of at least 107 poises at room temperature. 3.Transmission line of claim 2 in which said composition contains anaverage of at least 2000 carbon atoms in the main chain of the polymermolecule and is rigid at room temperature.
 4. Transmission line of claim1 in which essentially all the bonds between carbon atoms and terminalatoms are carbon-to-fluorine bonds.
 5. Transmission line of claim 4 inwhich said composition consists essentially of a fluorocarbon. 6.Transmission line of claim 5 in which said fluorocarbon is fluorinatedethylene propylene copolymer containing at least 10 percent of moietieswhich may be derived from each of the monomers, ethylene and propylene.7. Transmission line of claim 6 in which the average chain of saidcopolymer contains at least 103 monomeric units.
 8. Transmission line ofclaim 4 in which said composition consists essentially ofperfluoroalkylpolyethers, in which the average polymer chain contains atleast 103 monomeric units.
 9. Transmission line of claim 1 in which saidcomposition is normally liquid and has an average of up to 50 carbonatoms per molecule.